Although some deployments will specify driver and
connection pool parameters, the default values will be fine for most
applications.

Core Concepts

Connection

connection

An established channel of communication between a client and a server. The
client and the server may be on separate machines, on the same machine, or even
running in the same JVM. Often the connection is established using TCP/IP as
the transport of communication.

A database connection is used to allow the Java program, running in a JVM,
to communicate with a database server.

Connection Pool

connection pool

A set of connections maintained so that the connections can be reused when
there is a future need for the conneciton.

Connection pools are used to reduce the overhead of using a database.
Establishing a connection to the database is a costly operation. A connection
pool keeps a pool of open connections, each connection can be used for a time
as needed, and then released back to the pool. A connection that has been
released back to the pool can then be reused.

Connection pooling is especially important in server applications. The
overhead of opening a new connection for each new client request is too costly.
Instead, the database pool allows for a connection to be opened once and then
reused for many requests.

DataSource

DataSource

A JDBC term (and interface name) used for a factory that is used to obtain connections.

Resin provides an implementation of DataSource. Resin's
implementation of DataSource is a connection pool.

Driver

driver

An implemetation of a defined interface that hides the details of
communication with a device or other resource, such as a database.

A Driver provides an interface and is responsible for the communication
with the database. Every different database (i.e Oracle, MySQL) has their own
means of enabling communication from the client (in this case Resin and you
applications) and the database. The Driver provides a common interface that
hides the details of that communication.

Transaction

transaction

A transaction is used to mark a group of operations and provide a guarantee
that all of the operations happen, or none of them happen. Transactions
protect the integrity of the database.

Transactions are especially important in server applications where many
threads of processing may be interacting with the database at the same
time.

For a simple example, imagine a set of operations that reads a value,
calculates a new value, and then updates the database.

Imagine if one thread is performing this operation, and in the middle of
this read/calculate/update, another thread performs an update. The data that
the first thread obtained from the read and is using for the calculation and
update is no longer valid.

Placing the read/calculate/update operations in a transactions guarantees
that only one thread can perform those operations at a time, if a second thread
comes along and tries to perform the operation, it will have to wait for the
first thread to finish before it can begin.

A cache that holds prepared statements, a reused prepared statement avoids the overhead of the driver making the prepared statement

0

spy

A debugging aid, if true, generate info level log events that reveal the SQL that is used with the connections.

false

All times default to seconds, but can use longer time periods:

Time suffixes

SUFFIX

DESCRIPTION

s

seconds

m

minutes

h

hours

D

days

The class that corresponds to <database> is

Driver Configuration

driver

child of database

Configure a database driver. The driver is a class provided by the
database vendor, it is responsible for the communication with the database.

The jar file with the driver in it can be placed in WEB-INF/lib,
although it is often best to place your datbase driver's jar file in
$RESIN_HOME/lib/local/, which makes the driver available to all of
your web applications.

Choosing a driver class for <type>

Database vendors usually provide many different classes that are potential
candidates for type. The JDBC api has developed over time, and is now
being replaced by the more general JCA architecture. The driver you choose
depends on the options the vendor offers, and whether or not you need
distributed transactions.

JCA drivers

JCA is replacing JDBC as the API for database drivers. JCA is a much more
flexible approach that defines an API that can be used for any kind of
connection, not just a connection to a database. If a database vendor provides
a JCA interface, it is the best one to use.

A JCA driver implements ManagedConnectionFactory. When you
specify such a class for type, Resin will notice that it is a JCA driver
and take advantage of the added functionality that the JCA interface
provides.

The same JCA driver is used for both non-distributed and distributed
transactions

JDBC 2.0 - ConnectionPoolDataSource

JDBC 2.0 defined the interface ConnectionPoolDataSource. A
ConnectionPoolDataSource is not a connection pool, but it
does provide some extra information that helps Resin to pool the connection
more effectively.

A driver that implements ConnectionPoolDataSource is better than a
JDBC 1.0 driver that implements Driver.

JDBC 2.0 - XADataSource

JDBC 2.0 defined the interface XADataSource for connections that can
participate in distributed transactions. A distributed transaction is
needed when transactions involve multiple connections. For example, with two
different database backends, if the guarantees that transactions apply need to
apply to operations that occur on both databases within the same transaction,
distributed transactions are needed.

Distributed transactions are rarely needed, and few databases really support
them. Some vendors will provide XADataSource drivers even though
the database does not really support distributed transactions. Often,
XADataSource drivers are slower than their
ConnectionPoolDataSource counterparts.

XADataSource should only be used if distributed transactions
are really needed, and can probably be safely ignored for most
applications.

JDBC 1.0 - Driver

Driver is the original JDBC interface, and is the least desirable kind
of driver to use. Resin can still pool database connections using these drivers, but it will not be as efficient as the newer drivers.

Set driver properties with init-param

init-param is used to set properties of the database driver that are
specific to the driver and are not generic enough for resin to provide a
named configuration tag.

For example, MySQL drivers accept the useUnicode parameter, if
true the driver will use Unicode character encodings when handling
strings.

Pooling Configuration

Pooling configuration controls the behaviour of Resin's pooling of database
connections. For most applications and databases the only needed change is to
increase the max-connections value to meet high demand. Other pooling
parameters have defaults that are based on our years of experience with many
different databases in many different applications. Changes from the defaults
should only be done in response to specific problems, and with a good
understanding of how pooling works.

Reliability Configuration

ping

Resin's database pool can test if the pooled database connection
is still alive by configuring a ping query. This is typically only
necessary if the pooling parameters are changed from
their default values.

If the pool is configured with a long max-idle-time the database connection
may become stale if the database is restarted, or if the database is configured
with a shorter connection timeout value than the configuration of the Resin
pool. Normally when a database connection is returned to the pool it will wait
there until the next request or the idle-time expires. If the database goes
down in the meantime or closes the connection, the connection will become
stale. The ping configuration can test the database connection.

When pinging, Resin's DBPool will test a table specified with the
ping-table parameter before returning the connection to the application.
If the ping fails, the connection is assumed to be no good and a different
connection from the pool is returned. For a ping-table of BROOMS, Resin will
use the query select 1 from BROOMS where 1=0

<driver> list

If there is a pool of database servers available that can be used for database
operations, Resin can be configured with a list of <driver> tags.
Resin uses a round robin algorithm to cycle through the list of drivers when
obtaining connections. If a particular <driver> fails to provide a
connection, Resin continues the attempt to obtain a connection. If all of the
configured drivers fail to provide a connection the exception is propogated to
the caller.

<backup-driver> list

Drivers in a driver list can be marked as backups. The drivers configured with
<backup-driver> are used only if all of the drivers configured with
<driver> have failed.

Each time a new connection is needed Resin goes through the process of first
attempting to use one of the <driver> configured drivers to get a connection,
and if that fails then the <backup-driver> are used.
A new connection is needed from the driver if the pool of connections that is
maintained by Resin does not contain an idle connection. The
Pooling configuration and the usage pattern of the
application determine how often a connection is obtained from a driver. The
pooling configuration typically allows a single real connection to be reused by
the application many times.

The lifetime of a connection obtained from a <backup-driver> is determined by
the Pooling configuration, thus even if the main
<driver> becomes available again a connection previously obtained from a
<backup-driver> will continue to be used until it expires from the pool.

Obtaining and using a database connection

Getting the DataSource

The DataSource is a factory that is used to obtain
a connection. The DataSource is obtained using the <jndi-name> specified when configuring the database resource.

Ideally, the JNDI lookup of DataSource is done only once, the
DataSource obtained from the lookup can be stored in a member variable
or other appropriate place. The stored DataSource can then be used
each time a connection is needed. If it is not stored, there will be an impact
on performance from having to do the lookup each time you want to get a
connection.

Getting a Connection

A connection is obtained from the DataSource. The
connection is used as needed, and then released with a call to close() so that
Resin knows it is available for a subsequent request.

It is very important that the close() is always called, even
if there as an exception. Without the close(), Resin's database pool
can loose connections. If you fail to close() a connection, Resin does not know
that it is available for reuse, and cannot allocate it for another request.
Eventually, Resin may run out of connections.

Warning Always put a close() in a finally block, to guarantee that it is called.

The following example shows the use of a finally block that contains
the close(). Because the close() is in a finally block, it
will happen even if the code using the connection throws an exception.

Getting the underlying driver connection

UserConnection is a wrapper around the real driver connection, it
allows Resin to intercept the close() call and manage the underlying driver
connection.

In rare circumstances it is necessary to obtain the real connection returned by the driver.
Typically this is a requirement for situations where the driver provides a
specialized API that is not available with the standard JDBC API.

Protecting the database password

Resin provides facilities that allow you to plugin your own custom
code that returns a password to Resin. However any solution is vulnerable,
unless you require a person to type in a password every time Resin starts (or
restarts). Typically the security of the machine hosting Resin, and proper
permissions on the readability of the resin.conf file, are sufficient to
protect your database password.

The solution shown below is not really secure because you can disassemble the
Password code to get the decryption key, but it may be marginally better than
plaintext.

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